1. Technical Field
The present invention generally relates to mesoporous carbon materials comprising bi-functional catalysts, methods for making the same and devices containing the same.
2. Description of the Related Art
Metal-air batteries have a much higher theoretical specific energy than most available primary and rechargeable batteries, including metal oxide/carbon batteries. Metal-air batteries generally comprise an electrolyte interposed between positive and negative electrodes and are unique because the cathode active material is not stored in the battery. Instead, oxygen from the environment is reduced by catalytic surfaces in the air electrode, forming either an oxide or peroxide ion that further reacts with cationic species in the electrolyte. One such metal-air battery is the lithium-air battery. The positive electrode of a lithium-air battery generally comprises a lithium compound, such as lithium oxide or lithium peroxide, and serves to oxidize or reduce oxygen, while the negative electrode generally comprises a protected lithium foil which absorbs and releases lithium ions without allowing the lithium to react with the air or the electrolyte.
Lithium air batteries have a theoretical specific energy density of about 12 kWh/kg, which is the highest energy density of all electrochemical couples and compares favorably with the energy density of gasoline (˜12,300 kWh/kg). Thus, lithium-air batteries potentially enable electric vehicles with uncompromised range versus today's internal combustion engine powered automobiles. However, the theoretical energy density of the lithium-air electrochemical couple has yet to be achieved. Typically, the energy density is limited by the deposition of lithium peroxide (Li2O2) within the air electrode pores. The peroxide, which is the product of the air electrode reaction, is deposited in the electrode pores and prevents further current flow, thereby limiting the usefulness of the device.
While recent research has focused on increasing the energy density of lithium-air batteries, a lack of an appropriate material for the carbon-based electrode has prevented their development for practical applications. In this regard, the pore structure (e.g., pore volume, size and distribution) of the carbon materials in a lithium-air carbon electrode is a critical parameter that has yet to be optimized. The pore structure must have an appropriate catalytic surface and microstructure to facilitate the Li/O2 reaction and must accommodate sufficient amounts of reaction products (e.g., lithium peroxide) per gram of carbon to reduce clogging and inactivation of the pores. In addition, the capacity of carbon-based air electrodes increases with the mesopore volume of the carbon material but is not very sensitive to the bulk porosity. Finally, suitable carbon materials for use in metal-air batteries should be able to accommodate catalyst materials (e.g., metals and metal oxides) to enhance the reversibility of the metal-air reaction while maintaining the desired pore structure. Despite significant research in this area, carbon materials having such an optimized pore structure for use in lithium-air, and other metal-air batteries, and methods for making the same, are not currently known.
While significant advances have been made in the field, there continues to be a need in the art for improved mesoporous carbon materials for use in metal-air batteries, in particular lithium-air batteries, and other electrical energy storage devices, as well as for methods of making the same and devices containing the same. The present invention fulfills these needs and provides further related advantages.